Dual-control for controlling the liquid level in a continuous casting mold



Oct. 31, 1967 H. WILSON 3,349,834

J. DUAL-CONTROL FOR CONTROLLING THE LIQUID LEV IN A CONTINUOUS CASTING MOLD Filed Sept. 21, 1965 HMCH Ha SPaoReWm Claw/7' L/Qu/D LEI/1. Del M770 INVENTOR.

JAMES 19. Mme/v BY 3 1% United States Patent 6 3,349,834 DUAL-CONTROL FOR CONTROLLING THE LIQUID LEVEL IN A CONTINUOUS CAST- ING MOLD James H. Wilson, Franklin Township, Westmoreland County, Pa., assignor to United States Steel Corporation, a corporation of Delaware Filed Sept. 21, 1965, Ser. No. 489,018 3 Claims. (Cl. 164-155) This invention relates in general to a system for controlling the level of the molten metal in a continuous casting mold.

More particularly, this invention relates to a liquid level control system wherein a dual mode of operation is employed. The primary mode of operation controls the direction and speed of the motor driving the pinch rolls which withdraw the casting from the mold, as a function of the polarity and magnitude of a first error signal indicating deviation from a desired liquid level. The secondary mode of operation adds a control over the pour rate of metal into the mold when the pinch-roll drive speed becomes either greater or less than certain predetermined limits. This secondary mode of operation responds to a second error signal having a polarity and a magnitude that represent direction and magnitude of deviation in casting withdrawal rate (i.e. pinch-roll speed) from a desired predetermined rate.

This invention is an improvement on the invention described in the co-pending application Ser. No. 353,696 filed on Mar. 23, 1964, now Patent No. 3,300,820, issued Ian. 31, 1967, by John R. Tiskus and James H. Wilson. The system described therein employs a single error signal to simultaneously control the pinch-roll drive motor speed and a hydraulic system that in turn controls the stopper rod of a ladle and thus the pour rate into the mold. In this prior art technique, when the error signal indicates that the liquid level in the mold is too high then (1) the pinch-roll drive motor speed is increased so that the casting withdrawal rate increases and (2) the hydraulic system is pulsed so that the stopper rod position is changed in a direction that decreases the rate at which molten metal flows from a ladle into the mold. Essentially, the control function in the system of said patent is such that the casting withdrawal rate and ladle pour rate are regulated independently of one another, except to the extent that any change in the liquid level position will affect the control of both of these parameters.

The invention described herein provides an improvement on the control system of the patent in at least two particular respects.

First, the major purpose of this invention is to provide greater assurance that the casting withdrawal rate (pinchroll speed) is kept within a predetermined range so as not to disrupt other continuous functions of the casting operation such as the cooling of the casting and subsequent rolling.

A second specific improvement alforded by this invention is in providing a more stable liquid level through control of a single parameter (casting withdrawal rate) over most of the liquid level deviation range that is likely to be encountered.

A third advantage of this invention arises from the fact that the control system provided will maintain the proper liquid level in the mold over most of the operating conditions that are expected to be encountered even though fiow control into the mold shouldsuddenly be lost because of a jammed ladle stopper rod or the freezing of the ladle nozzle.

In brief, the system of this invention employs a known liquid level deviation detector to establish a first error signal whose polarity and magnitude represent the direction and magnitude of deviation from a predetermined desired level of the metal liquid level in a continuous casting mold. This first error signal is employed to control the speed of the pinch-roll drive motor, thereby controlling the rate at which the metal casting is withdrawn from the mold. In this fashion, the metal casting withdrawal rate is adjusted to maintain the desired liquid level. If the liquid level goes higher than desired, then the withdrawal rate increases and if the liquid level drops below the desired level, then the withdrawal rate decreases. A second error signal is generated whose polarity and magnitude represent the direction and magnitude of deviation of pinch-roll drive motor speed from a desired optimum speed. The optimum speed is dictated by the optimum casting withdrawal rate for proper operation of the entire continuous casting system which includes considerations of optimum casting cooling rates and optimum rolling rates. This second error signal is applied to a known control circuit to pulse a solenoid controlled hydraulic valve which in turn affects the position of a stopper rod in a ladle. This, of course, determines the rate at which molten metal is poured into the mold. Crucial to the invention is the incorporation of a means such as a dead band control to make sure that the second error signal is applied to the pour rate control system only when it exceeds a predetermined value on either side of zero. The limits of this dead band are set so that the second error signal is effective to change the rate at which liquid is poured from the ladle into the mold only when the casting withdrawal rate deviates from optimum by a predetermined excessive amount.

Other objects and purposes of the improvement that constitutes this inventiton will become clear from the following detailed description, referring to the accompanying drawing in which:

The sole figure is a block and electrical schematic diagram of the invention.

With reference to the drawing, the device of this invention employs a potentiometer 11 connected across a battery 13 so that the slider arm 12 can be adjusted to pick off a desired reference voltage E. This reference voltage E is selected to equal the normal magnitude of the voltage signal output E, from a tachometer 15 which is connected to the pinch-roll drive motor 79. The motor 79 has its speed regulated by acircuit 75, which circuit may be exactly the same as the pinch-roll speed regulator circuit described in the above patent. The circuit 75 in turn is controlled by an error signal E which is provided by a liquid level deviation detector 40, which detector may be the type employed in the patent. The error signal E, has a polarity and magnitude that reflect, respectively, the direction of deviation and magnitude of deviation of the liquid level in the mold from a predetermined desired level.

When the motor 79 is operated at optimum desired spee then the difference between the tachometer output E and the preselected reference voltage E will 'be zero. When the pinch-roll speed is not at the desired value, the voltage E produced by the tachometer generator 14 is not equal to the predetermined reference voltage B and an error voltage E is produced. This error voltage E has a polarity indicating the direction of casting speed deviation from the desired speed and a magnitude proportional to the magnitude of the casting speed deviation from the desired or optimum speed.

A liquid level control circuit 45 is employed that \may be identical with the liquid level control circuit 45 described in the above patent and it provides output signals E and E that in turn vary the position of the stopper rod in a ladle so as to effect the rate at which molten liquid is poured into the mold. Where the switch 19 is in the position to engage terminals T the liquid level control circuit 45 will be responsive to the secondary error signal E' However, the response of this liquid level control circuit 45 to the secondary error signal E is effected by the limit control circuit 18.

This limit control circuit 18 assures that the liquid level control circuit 45 will respond to the secondary error signal E only when the magnitude of the secondary error signal E falls outside of a band of values, which band of values is selected to represent the tolerable limits on the casting withdrawal rate.

The limit circuit 18 employs two resistors 20* and 21 both have slider arms 23 and 24-, respectively. These resistors 2t and 21 are connected in series with each other and the series combination is connected across batteries 26 and 27 so arranged as to provide a positive voltage at the tap 23 and a negative voltage at the tap 24. The variable taps 23 and 24 are set at whatever voltages represent the value of the secondary error signal E when the casting withdrawal rate is at its maximum and minimum tolerable limits. In this fashion the range during which the secondary error signal E is inoperable is determined by the setting of the slider arms 23 and 24. The diode 30 prevents a positive voltage at the terminal T from causing current to fiow through resistor 20 and, similarly, the diode 31 prevents a negative voltage at the terminal T from causing current to flow through resistor 21. In this fashion the range within which the secondary error signal E is operable is determined by the setting of the slider arms 23 and 24. The secondary error signal E as seen by the control circuit has a center band removed from it.

With the above in mind, it can readily be seen that in operation the liquid level control circuit 45 is never brought into operation until the secondary error signal E' is either sufiiciently negative or suificiently positive to indicate that the casting withdrawal rate has exceeded tolerable limits. Thus with the improvement of this invention the primary control over liquid level deviation is through control of the casting withdrawal rate. However, excessively high and excessively low withdrawal rates are undesirable for other system reasons. Thus this invention throws an additional control into operation when, and only when, the liquid level cannot be adequately controlled by means of variation of the casting withdrawal rate Within whatever tolerable limits the entire continuous casting system will permit.

The terminals T are provided so that when the switch 19 is thrown to a position engaging them, the liquid level control system operates exactly as is described in the above patent. The improvements provided by this invention are operative only when the dual position switch 19 is thrown to be in contact with the terminals T Although this invention has been described in one detailed embodiment and in connection with a continuous casting mold wherein a stopper rod controlled ladle is employed to pour liquid metal into the mold, it is to be understood that certain variations in the detailed structure of the invention are within the skill of the art and are intended to be encompassed within the following claims. In particular, this invention may be employed in connection with a continuous casting operation wherein a tundish or a vacuum bottle is employed as the vessel from which liquid is poured into the mold. In both of these cases, the invention will be seen most clearly to be one wherein a casting withdrawal rate control provides a fine control over liquid level and a pour rate control provides a gross control over liquid level.

What is claimed is:

1. In a system for controlling the liquid level in a continuous casting mold, said system including means to provide a first error signal having the characteristic that its polarity indicates the direction of liquid level deviation from a predetermined level and its magnitude indicates the extent of deviation of liquid level from said predetermined level, said system also including a pour rate regulating means for varying the rate of pour of molten metal into said mold, said pour rate regulating means being responsive to an input control signal such that an input control signal having a first polarity will cause the pour rate to decrease and an input signal having a second polarity will cause the pour rate to decrease, said system also including a pinch-roll drive for withdrawing a casting from said mold, said pinch-roll drive being responsive to said first error signal so that a first error signal having a first polarity and a magnitude greater than a first predetermined value will cause the withdrawal rate of said casting to increase and an error signal having a second polarity and a magnitude greater than a second predetermined value will cause the withdrawal rate of said casting to decrease, the improvement comprising:

(A) a tachometer generator coupled to said pinch-roll drive motor to provide a tachometer signal having a magnitude representing the speed of said motor,

(B) a potentiometer having a sliding arm to provide a pre-selected reference signal representing the value of said tachometer signal when the pinch-roll drive motor is running at a speed that provides a desired casting withdrawal rate,

(C) means to compare said tachometer signal to said reference signal to provide a second error signal having a polarity and magnitude representing respec tively the direction and magnitude of deviation of said casting withdrawal rate from a predetermined desired rate,

(D) means for coupling said second error signal to said pour rate regulating means as the input control signal for said pour rate regulating means, and

(E) a pair of biased diodes coupled to said second error signal to provide a dead band in the response of said pour rate regulating :means to said second error signal by creating biases in both polarities in opposition to said second error signal so that said second error signal is rendered ineffective when the absolute magnitude of said second error signal is less than the corresponding bias magnitude.

2. In a system for controlling the liquid level in a continuous casting mold, said system including means to provide a first error signal having the characteristic that its polarity indicates the direction of liquid level deviation from a predetermined level and its magnitude indicates the extent of deviation of liquid level from said predetermined level, said system also including a pour rate regulating means for varying the rate of pour of molten metal into said mold, said pour rate regulating means having first and second input terminals and being responsive to an input control signal at said terminals such that an input control signal having a first polarity Will cause the pour rate to decrease and an input signal having a second polarity will cause the pour rate to decrease, said system also including a pinch-roll drive for withdrawing a casting from said mold, said pinch-roll drive being responsive to said first error signal so that a first error signal having a first polarity and a magnitude greater than a first predetermined value will cause the withdrawal rate of said casting to increase and an error signal having a second polarity and a magnitude greater than a second predetermined value will cause the withdrawal rate of said casting to decrease, the improvement comprising:

(A) means coupled to said pinch-roll drive motor to provide a speed voltage having a magnitude representing the speed of said motor, said speed voltage being coupled to said first input terminal of said pour rate regulating means,

(B) means to provide a pre-selected reference voltage equal to the value of said speed voltage when the pinch-roll drive motor is running at a speed that provides a desired casting withdrawal rate,

(C) means to provide a positive bias voltage having a first predetermined magnitude,

(D) a first diode having its cathode connected to said positive bias voltage means and its anode connected to said second input terminal of said pour rate regulating means,

(E) means to provide a negative bias voltage having a second predetermined magnitude,

(F) a second diode having its anode connected to said negative bias voltage means and its cathode connected to said second input terminal of said pour rate regulating means, and

(G) means to couple said reference voltage to said cathode of said first diode and to said anode of said second diode, whereby current will flow at said second terminal of said pour rate regulating means when the difference between said speed voltage and said reference voltage exceeds in absolute magnitude the the absolute magnitude of said bias voltage means.

3. A system for controlling the liquid level in a cntinuous casting mold comprising:

(A) means to provide a first error signal having the characteristic that its polarity indicates the direction of liquid level deviation from a predetermined level and its voltage magnitude indicates the extent of deviation of liquid level from said predetermined level,

(B) a pour rate regulating means for varying the rate of pour of molten metal into said mold, said pour rate regulating means being responsive to an input control signal such that an input control signal having a first polarity will cause the pour rate to decrease and an input signal having a second polarity will cause the pour rate to decrease,

(C) a pinch-roll drive for withdrawing a casting from said mold, said pinch-roll drive being responsive to said first error signal so that a first error signal having a first polarity and a magnitude greater than a first predetermined value will cause the withdrawal rate of said casting to increase and an error signal having a second polarity and a magnitude greater than a second predetermined value will cause the withdrawal rate of said casting to decrease,

(D) tachometer means coupled to said pinch-roll drive motor to provide a speed voltage having a magnitude proportional to the speed of said motor,

(E) means to provide a preselected reference voltage equal to the value of said speed voltage when the pinch-roll drive motor is running at a speed that provides a desired casting withdrawal rate,

(F) means to take the algebraic dilference between said speed voltage and said reference voltage to provide a second error signal having a polarity and voltage magnitude representing respectively the direction and magnitude of deviation of said casting withdrawal rate from a predetermined desired rate,

(G) means for coupling said second error signal to said pour rate regulating means as the input control signal for said pour rate regulating means, and

(H) bias means coupled to said second error signal to provide a dead band in the response of said pour rate regulating means to said second error signal by creating biases in both polarities in opposition to said second error signal so that said second error signal is rendered ineffective when the absolute magnitude of said second error signal is less than the corresponding bias magnitude.

References Cited UNITED STATES PATENTS 2,709,284 5/1955 Evans et al. l64281 X 2,753,605 7/1956 Carleton 164-155 2,824,346 2/1958 Osborn 164-155 3,204,460 9/1965 Milnes 164277 X 3,300,820 1/1967 Tiskus et al. 164-455 3,307,228 3/1967 Scribner 164-154 X J. SPENCER OVERHOLSER, Primary Examiner. R. S. ANNEAR, Assistant Examiner. 

1. IN A SYSTEM FOR CONTROLLING THE LIQUID LEVEL IN A CONTINUOUS CASTING MOLD, SAID SYSTEM INCLUDING MEANS TO PROVIDE A FIRST ERROR SIGNAL HAVING THE CHARACTERISTIC THAT IS POLARITY INDICATES THE DIRECTION OF LIQUID LEVEL DEVIATION FROM A PREDETERMINED LEVEL AND ITS MAGNITUDE INDICATES THE EXTENT OF DEVIATION OF LIQUID LEVEL FROM SAID PREDETERMINED LEVEL, SAID SYSTEM ALSO INCLUDING A POUR RATE REGULATING MEANS FOR VARYING THE RATE OF POUR OF MOLTEN METAL INTO SAID MOLD, SAID POUR RATE REGULATING MEANS BEING RESPONSIVE TO AN IMPUT CONTROL SIGNAL SUCH THAN AN INPUT CONTROL SIGNAL HAVING A FIRST POLARITY WILL CAUSE THE POUR RATE TO DECREASE AND AN INPUT SIGNAL SUCH A SECOND POLARITY WILL CAUSE THE POUR RATE TO DECREASE, SAID SYSTEM ALSO INCLUDING A PINCH-ROLL DRIVE BEING REA CASTING FROM SAID MOLD, SAID PINCH-ROLL DRIVE BEING RESPONSIVE TO SAID FIRST ERROR SIGNAL SO THAT A FIRST ERROR SIGNAL HAVING A FIRST POLARITY AND A MAGNITUDE GREATER THAN A FIRST PREDETERMINED VALUE WILL CAUSE THE WITHDRAWAL RATE OF SAID CASTING TO INCREASE AND AN ERROR SIGNAL HAVING A SECOND POLARITY AND A MAGNITUDE GREATER THAN A SECOND PREDETERMINED VALUE WILL CAUSE THE WITHDRAWAL RATE TO SAID CASTING TO DECREASE, THE IMPROVEMENT COMPRISING: (A) A TACHOMETER GENERATOR COUPLED TO SAID PINCH-ROLL DRIVE MOTOR TO PROVIDE A TACHOMETER SIGNAL HAVING A MAGNITUDE REPRESENTING THE SPEED OF SAID MOTOR, (B) A POTENTIOMETER HAVING A SLIDING ARM TO PROVIDE A PRE-SELECTED REFERENCE SIGNAL REPRESENTING THE VALUE OF SAID TACHOMETER SIGNAL WHEN THE PINCH-ROLL DRIVE MOTOR IS RUNNING AT A SPEED THAT PROVIDES A DESIRED CASTING WITHDRAWAL RATE, (C) MEANS TO COMPARE SAID TACHOMETER SIGNAL TO SAID REFERENCE SIGNAL TO PROVIDE A SECOND ERROR SIGNAL HAVING A PLORALITY AND MAGNITUDE REPRESENTING RESPECTIVELY THE DIRECTION AND MAGNITUDE OF DEVIATION OF SAID CASTING WITHDRAWAL RATE FROM A PREDETERMINED DESIRED RATE, (D) MEANS FOR COUPLING SAID SECOND ERROR SIGNAL TO SAID POUR RATE REGULATING MEANS AS THE IMPUT CONTROL SIGNAL FOR SAID POUR RATE REGULATING MEANS, AND (E) A PAIR OF BIASED DIODES COUPLED TO SAID SECOND ERROR SIGNAL TO PROVIDE A DEAD BAND IN THE RESPONSE OF SAID POUR RATE REGULATING MEANS TO SAID SECOND ERROR SIGNAL BY CREATING BIASES IN BOTH POLARITIES IN OPPOSITION TO SAID SECOND ERROR SIGNAL SO THAT SAID SECOND ERROR SIGNAL IS RENDERED INEFFECTIVE WHEN THE ABSOLUTE MAGNITUDE OF SAID SECOND ERROR SIGNAL IS LESS THAN THE CORRESPONDING BIAS MAGNITUDE. 